Why does an array with a variable give the wrong result?

Question

PHP

$d = ("1, 2, 3");
$array = array($d);
print_r($array);

Why does an array with a variable not work, we get the following result:

Array
(
    [0] => 1, 2, 3
)

Answer 1

What do you want to receive? Your array consists of one string with numbers separated by commas. You get an array of arrays.

Answer 2

Then the question is different, how to split the enumeration into lines?
like in this example but with $a variable inside array which contains 1, 2 ,3 ?
$array = array(1, 1, 1, 1,);
print_r($array);